1. Field of the Invention
The present invention concerns a method and device for determining a force exerted on a wheel of a motor vehicle by a ground surface supporting the wheel, along a longitudinal direction of the wheel.
2. Description of Related Art
On-board electronic systems are known, which are designed to assist the driver of a vehicle when he is in various types of difficult situations. The best known are without doubt the ABS anti-blocking systems and the ESP electronic regulation system for the dynamic behaviour of the vehicle.
An ABS system acts on the braking system to prevent blocking of the wheels during emergency braking. An ESP system also acts on the braking system, and if necessary on the engine control system, so as to improve a vehicle's path in a situation of dangerous cornering, for example when a tight curve has been entered at too high a speed, i.e. to counteract any tendency of the vehicle to oversteer or understeer.
A common point of such systems is to aim to reinforce the road-holding of a motor vehicle, more precisely to seek to adapt the handling of the vehicle so that the longitudinal and/or lateral acceleration required does not exceed the amount of force which the road or surface on which the vehicle is moving can effectively transmit to the vehicle via its tires. This can be done on the one hand by adapting the amount of force and on the other hand by adapting the handling parameters, such as the braking force, the running of the engine or the steering angle of the wheels.
Thus, an ABS system controls the braking force applied to the wheel to maintain the longitudinal force exerted by the road on the tire at a maximum allowable level. In a known way, this control is based on a measurement of the sliding speed Vg of the tire, defined for example by: Vg=|Rω−Ve| where ω denotes the wheel rotation speed, R the outer radius of the wheel and Ve the overall speed of the vehicle.
FIG. 5 shows the variation of the coefficient of friction μ between the tire and the road as a function of the sliding speed Vg. The figure shows that there is an optimum sliding speed Vg0 for which the coefficient of friction μ is maximum. If the load supported by the wheel is taken as constant, that optimum sliding speed Vg0 also maximises the force of the road on the tire, which is proportional to the coefficient of friction μ.
If, during braking with the aid of a working brake, the sliding speed Vg of the wheel exceeds Vg0, the force exerted by the road on the wheel starts decreasing, causing the vehicle's deceleration to tend to decrease while the deceleration of the wheel tends to increase. This is because during braking, the longitudinal contact force between the ground and the wheel is both a force that decelerates the vehicle and a force driving the wheel which is opposed to the braking force applied by the working brake. The result of these two tendencies is that the sliding speed increases, which in turn accentuates the reduction of the friction coefficient. Under these conditions, unless the braking force applied to the wheel is relaxed completely, it can be seen that the wheel will rapidly tend towards complete blockage, which in fact is a situation of stable equilibrium for the wheel. Thus, to achieve the shortest possible braking, the braking force should be as large as possible without the sliding speed Vg exceeding Vg0.
Consequently, as a safety measure ABS systems are classically adjusted to operate in a sliding speed range S located substantially below Vg0, as shown in FIG. 5, so as to avoid the critical zone Vg>Vg0. Although it might seem theoretically possible to extend the range of operation of the system up to Vg=Vg0, this cannot be done in practice because of the variability of Vg0 and errors in the measurement of Vg. Thus, the known ABS systems lead to a force exerted on the wheel by the ground which is below its optimum value.
To overcome this disadvantage, it would be desirable to base a braking force control algorithm, for example for an ABS system, not on the value of the sliding speed but directly on the value of the force exerted by the ground on the wheel, which presupposes that this value is known.